In today's wireless communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible technologies for wireless communication. A wireless communications network comprises radio base stations providing radio coverage over at least one respective geographical area forming a cell. The cell definition may also incorporate frequency bands used for transmissions, which means that two different cells may cover the same geographical area but using different frequency bands. Wireless devices, also referred to herein as User Equipments, UEs, mobile stations, and/or wireless terminals, are served in the cells by the respective radio base station and are communicating with respective radio base station. The wireless devices transmit data over an air or radio interface to the radio base stations in uplink, UL, transmissions and the radio base stations transmit data over an air or radio interface to the wireless devices in downlink, DL, transmissions.
Long Term Evolution, LTE, is a Frequency Division Multiplexing technology wherein Orthogonal Frequency Division Multiplexing, OFDM, is used in DL transmissions from a radio base station to a wireless device and Discrete Fourier Transform spread, DFT-spread OFDM is used in UL transmissions from a wireless device to a radio base station. FIGS. 1-3 provide an overview of LTE downlink transmissions.
FIG. 1 illustrates the basic LTE physical resource which may be seen as a time-frequency grid, where each resource element corresponds to one subcarrier during one OFDM symbol interval (on a particular antenna port). As shown in FIG. 1, in the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of 1 ms as illustrated in FIG. 2. A subframe is divided into two slots, each of 0.5 ms time duration.
As shown in FIG. 2, the resource allocation in LTE is described in terms of resource blocks, where a resource block corresponds to one slot in the time domain and 12 contiguous 15 kHz subcarriers in the frequency domain. Two in time consecutive resource blocks represent a resource block pair and correspond to the time interval upon which scheduling operates.
Transmissions in LTE are dynamically scheduled in each subframe where the base station transmits downlink assignments/uplink grants to certain wireless devices via the Physical Downlink Control Channel, PDCCH. The PDCCHs are transmitted in the first OFDM symbol(s) in each subframe and spans (more or less) the whole system bandwidth. Alternatively or additionally, enhanced PDCCHs, ePDCCHs, may be employed in LTE in which extended control regions are reserved for the transmission of downlink assignments/uplink grants. The ePDCCH is frequency multiplexed with scheduled data transmissions and uses dedicated demodulation reference signals for enhanced beamforming support. Hence, a UE that has decoded a downlink assignment, carried by a PDCCH and/or ePDCCH, knows which resource elements in the subframe that contain data aimed for the wireless device. Similarly, upon receiving an uplink grant, the wireless device knows which time/frequency resources it should transmit upon. In LTE downlink, data is carried by the Physical Downlink Shared Channel, PDSCH, and in the uplink the corresponding channel is referred to as the Physical Uplink Shared Channel, PUSCH.
Demodulation of sent data requires estimation of the radio channel which is done by using transmitted Reference Symbols, RS, i.e. symbols known by the receiver. In LTE, Cell-specific Reference Symbols, CRS, are transmitted in all downlink subframes and in addition to assist downlink channel estimation they may also be used for mobility measurements performed by the wireless devices. LTE also supports UE specific RS aimed only for assisting channel estimation for demodulation purposes. FIG. 3 illustrates how the mapping of physical control/data channels and signals may be done on resource elements within a downlink subframe. In this example, the PDCCHs occupy the first out of three possible OFDM symbols, so in this particular case the mapping of data could start already at the second OFDM symbol. Since the CRS is common to all wireless devices in the cell, the transmission of CRS cannot be easily adapted to suit the needs of a particular wireless device. This is in contrast to UE specific RS which means that each wireless device has RS of its own placed in the data region of FIG. 3 as part of PDSCH.
CSI-RS
As of LTE Release 10, a new RS concept was introduced with separate UE specific RS for demodulation of PDSCH and RS for measuring the channel for the purpose of Channel State Information, CSI, feedback from the wireless device. The latter is referred to as CSI-RS. CSI-RS are not transmitted in every subframe and they are generally sparser in time and frequency than RS used for demodulation. CSI-RS transmissions may occur every 5th, 10th, 20th, 40th, or 80th subframe according to an RRC configured periodicity parameter and an RRC configured subframe offset.
A wireless device operating in connected mode can be requested by the base station to perform CSI reporting, e.g. reporting a suitable rank indicator (RI), one or more precoding matrix indices (PMIs) and a channel quality indicator (CQI). Other types of CSI are also conceivable including explicit channel feedback and interference covariance feedback. The CSI feedback assists the network in scheduling, including deciding the subframe and RBs for the transmission, which transmission scheme/precoder to use, as well as provides information on a proper user bit rate for the transmission, i.e. link adaptation. In LTE, both periodic and aperiodic CSI reporting is supported. In the case of periodic CSI reporting, the wireless device reports the CSI measurements on a configured periodical time basis on the Physical Uplink Control Channel, PUCCH, whereas with aperiodic reporting the CSI feedback is transmitted on the PUSCH at pre-specified time instants after receiving the CSI grant from the base station. With aperiodic CSI reports, the base station may thus request CSI reflecting downlink radio conditions in a particular subframe.
FIGS. 4-6 shows a detailed illustration of which resource elements within a resource block pair may potentially be occupied by the new UE specific RS for demodulation and the CSI-RS. The CSI-RS utilizes an orthogonal cover code to overlay two antenna ports on two consecutive REs. As seen, many different CSI-RS patterns are available. For the case of 2 CSI-RS antenna ports, there are twenty different patterns within a subframe. The corresponding number of patterns is 10 and 5 for 4 and 8 CSI-RS antenna ports, respectively. For TDD, some additional CSI-RS patterns are available. A pattern may in LTE Release 10 correspond to 1, 2, 4, or 8 CSI-RS antenna ports.
Subsequently in this disclosure, the term CSI-RS resource is used to refer to a selection of resource elements corresponding to a CSI-RS. In FIGS. 4-6, for example, the resource elements corresponding to a CSI-RS resource share the same shading. In such a case, a resource corresponds to a particular pattern present in a particular subframe. Thus two different patterns in the same subframe or the same CSI-RS pattern but in different subframes in both cases constitute two separate CSI-RS measurement resources. In LTE Release 10, a CSI-RS resource can alternatively be thought of being pointed out by a combination of “resourceConfig” and “subframeConfig” which are configured by higher layers.
The CSI-RS patterns may also correspond to so-called zero-power CSI-RS, also referred to as muted REs. Zero-power CSI-RS corresponds to a CSI-RS pattern whose REs are silent, i.e. there is no transmitted signal on those REs. In other words, the zero-power CSI-RS are such that wireless device should not assume PDSCH energy in those REs. There may, and typically are, signals on those REs but those REs are not intended for the wireless device. These silent patterns are configured with a resolution corresponding to the four antenna port CSI-RS patterns. Hence, the smallest unit to silence corresponds to four REs.
One purpose of zero-power CSI-RS is to raise the SINR for CSI-RS in a cell by configuring zero-power CSI-RS in interfering cells so that the REs otherwise causing the interference are silent, which will improve channel estimation. Another purpose is to enable the wireless device to perform interference measurements. Thus, a CSI-RS pattern in a certain cell is matched with a corresponding zero-power CSI-RS pattern in interfering cells.
Raising the SINR level for CSI-RS measurements is particularly important in applications such as Coordinated Multi Point, CoMP, transmissions or in heterogeneous deployments. In CoMP, the wireless device is likely to need to measure the channel from non-serving points and interference from the much stronger serving point. Zero-power CSI-RS is also needed in heterogeneous deployments where zero-power CSI-RS in the macro-layer is configured so that it coincides with CSI-RS transmissions in the pico-layer. This avoids strong interference from macro nodes when wireless devices measure the channel to a pico node.
The PDSCH is mapped around the REs occupied by CSI-RS and zero-power CSI-RS in which case it is important that both the network and the wireless device are assuming the same CSI-RS/zero power CSI-RS configuration or else the wireless device is unable to decode the PDSCH in subframes containing CSI-RS or their zero-power counterparts. However, in some cases, the network and the wireless device may not be assuming the same CSI-RS/zero power CSI-RS configuration. For example, the network may transmit PDSCH (not intended for the wireless device) or CSI-RS in REs defined for the wireless device as zero-power, which may very well be a desired way to operate. However, it is important that the network is operating in the intended way implied by how the wireless device is configured.
Wireless device using Transmission Mode 10, TM10, may also be configured to report CSI for multiple CSI-processes, wherein each CSI-process may have different CSI-measurement resources. A CSI process comprises a CSI-RS measurement resource and a CSI Interference Measurement, CSI-IM, resource.
CSI Feedback for CoMP
To assist scheduling and link adaptation when performing CoMP, it is useful to let the wireless device feedback CSI corresponding to the channels of multiple transmission points back to the network node. Such feedback allows the network node to take the multiple transmission points into account, i.e. to assess the impact that these will have on the performance of scheduling a wireless device on a certain resource and with a certain precoder. This may then be exploited for devising efficient scheduling strategies across multiple transmission points.
In a typical CoMP scenario, two or more network nodes, which also may be referred to as Transmission Points or TPs hereinafter, may perform transmissions in a coordinated manner to a number of wireless devices in a wireless communications network. In such a CoMP scenario, accurate CSI for an intended transmission from one network node to one wireless device is a CSI that resembles the interference situation for that particular transmission. However, the interference situation for this particular transmission mainly depends on if other network nodes are also transmitting at the same time or if they are silent, i.e. muted.
For example, assume three network nodes, TP1, TP2, TP3, that are part of a coordinated cluster configured to perform CoMP transmissions, whereby a transmission scheduler for the CoMP transmissions, e.g. located in one of the three network nodes, requires to obtain accurate CSI for a transmission from the network node TP1 to a wireless device. In this case, an accurate CSI would then be a CSI according to one of the following CSI hypothesis:                CSI1: TP2 and TP3 both transmit        CSI2: TP2 transmit and TP3 muted        CSI3: TP2 muted and TP3 transmit        CSI4: TP2 and TP3 muted        
To get a CSI for all these CSI hypothesis, the wireless device is required to be configured with four CSI processes, whereby each CSI-process is configured with a CSI-IM resource on which the desired interference may be estimated. However, such a scheme to obtain accurate CSI within a coordinated cluster of network nodes configured to perform CoMP transmissions will consume a large amount of CSI-RS measurement resources in the wireless communications network.